Summary: Sera and saliva of about 67% of 300 rabbits contained a substance which was capable of inhibiting the reaction between cattle anti-J serum and J-positive cells. The presence of this J-like substance in rabbit serum and presumably in saliva is controlled by an autosomal dominant gene. A J-like substance was found also in a nondialyzable fraction of rabbit urine. Rabbits lacking the J-like substance when injected with saliva from cattle containing J produced specific immune anti-J. Naturally occurring antibodies of rabbits did not show seasonal variation, but this could possibly be explained by the constant climatic conditions under which the rabbits were kept.